In the field of medicine the safe handling of medical specimens has assumed a new criticality.
The onset of acquired immune deficiency (AIDS) stemming from the highly contagious HIV virus, combined with greater awareness of the carcinogenic pathology of substances in common medical use, such as formaldehyde has created an urgent demand for sample storage jars or containers that are of reasonable cost, not susceptible to breakage or leakage, are readily sealed and unsealed, and are suited to disposal by incineration.
The inherent dangers of breakage of glass containers, and the probability of generating dangerously sharp shards upon the occurrence of breakage that are susceptible to spreading infection, allied with the difficulty of ready, economical, safe and permanent disposal of glass containers militates against their continued use.
In the vast array of the plastics family polypropylene possesses many of the sealing characteristics required in the field of medical use, possessing as it does the qualities of: high resistance to leakage; resilience (i.e. non-frangible under impact); highly resistant to chemical or biological infiltration; moderate cost; substantially inert; stable at the temperatures of normal use; may be relatively safely incinerated.
However, polypropylene possesses one inherent characteristic which generally renders it unsuitable for use where high integrity seals are necessary. That characteristic is the tendency, when elastically deformed at a pressure point constituting a seal interface, for the polypropylene to gradually "draw back" at the point of contact, thereby reducing the contact pressure at the seal interface, to thus impair or destroy the integrity of the seal.
In the medical field of use, this characteristic has substantially negated the possible use of polypropylene containers, at least for containment of potentially contaminated tissue samples.
In the case of pressurized containers such as glass bottles and plastic containers for carbonated drinks, sealed by use of screw caps, U.S. Pat. No. 4,206,852 Dunn et al., issued June 1980, teaches the use of a plastic cap, preferably of polypropylene.
The internal shaped recess in the crown of the Dunn et al. cap is inset, radially, having a deformable centre crown portion of the closure cap that is acted upon by internal gas pressure present within the bottle, to exert a deforming sealing force against a narrow annular sealing band where the cap bears against the interior of the bottle neck, using a so-called "Belleville spring" effect to generate the desired deforming force in response to applied gas pressure.
U.S. Pat. No. 4,771,905 Perne et al., issued September 1988, operates in similar fashion, the deformed cap bearing radially outwardly against a narrow inner surface of the convoluted bottle neck. Again, internal gas pressure provides the deforming force.
U.S. Pat. No. 4,341,320 Libit, issued July 1982, also is applied to necked containers, wherein the seal is applied internally of the bottle neck as a consequence of internal gas pressure. A complex, axially deformable crown portion also is involved, as a tamper tell-tale.
Other forms of non-screwed lip seals are to be found in U.S. Pat. Nos. 3,165,227 Crowell et al., issued Jan. 1965; 3,692,208 Croyle et al., issued September 1972; and 4,844,961 Akao, issued July 1989, which rely upon deforming respective members of the container/closure combination.